Exciton diffusion in WSe2 monolayers embedded in a van der Waals heterostructure

TL;DR

This study combines spatially-resolved and time-resolved photoluminescence to measure exciton diffusion in hBN-encapsulated WSe2 monolayers, revealing significantly enhanced mobility and distinct behaviors of various exciton species at different temperatures.

Contribution

It provides the first detailed measurement of exciton diffusion lengths and coefficients in encapsulated WSe2 monolayers, highlighting the impact of encapsulation on exciton transport properties.

Findings

Exciton diffusion length at 300 K is 0.36 μm.

Diffusion coefficient is 14.5 cm²/s.

Dark neutral excitons have a diffusion length of 1.5 μm.

Abstract

We have combined spatially-resolved steady-state micro-photoluminescence ($\mu$PL) with time-resolved photoluminescence (TRPL) to investigate the exciton diffusion in a WSe$_2$ monolayer encapsulated with hexagonal boron nitride (hBN). At 300 K, we extract an exciton diffusion length $L_X= 0.36\pm 0.02 \; \mu$m and an exciton diffusion coefficient of $D_X=14.5 \pm 2\;\mbox{cm}^2$/s. This represents a nearly 10-fold increase in the effective mobility of excitons with respect to several previously reported values on nonencapsulated samples. At cryogenic temperatures, the high optical quality of these samples has allowed us to discriminate the diffusion of the different exciton species : bright and dark neutral excitons, as well as charged excitons. The longer lifetime of dark neutral excitons yields a larger diffusion length of $L_{X^D}=1.5\pm 0.02 \;\mu$m.